Scientists have found that the brains of left-handed people are shaped
differently than those of right-handed people. Right-handers usually have
asymmetric brains; their left cerebral hemispheres tend to be larger than
their right hemispheres. In particular, Broca's and Wernicke's areas, two
regions involved with language, are likely to be much larger in the left
hemisphere than in the right. In contrast to right-handers, left-handers
generally have more symmetrical brains, with similarly sized language
centers in each hemisphere.

One possible explanation for this observation is that most right-handers
have a "right-shift" set of genes. When these genes are active, the
person will be right-handed and most likely will have a larger left
cerebral hemisphere. If a person does not have active forms of these
"right-shift" genes, there will be no genetic control over handedness or
brain symmetry. These people can be either left- or right-handed, and
their brains will be more symmetrical. A study by Daniel Geschwind and his
colleagues at the University of California in Los Angeles adds support to
the theory of genetic control over brain symmetry.

Geschwind and his team measured the volume of the left and right cerebral
cortex in identical male twins and compared these volumes to handedness.
Consistent with previous research, the left-handers had more symmetrical
brains than the right-handers. Additionally, they compared the hemispheric
volumes in each set of twins and found that right-handed twin sets were
more likely to have similarly shaped brains than were twin sets in which
at least one man was left-handed. These results support the "right-shift"
genetic theory. It seems that there is genetic control over the asymmetry
present in the brains of most right-handers. The loss of asymmetry in
left-handed people may reflect a lack of this genetic
control.